More and more motor vehicles have, for safety purposes, monitoring systems comprising sensors mounted on each of the wheels of the vehicle, dedicated to measuring parameters, such as pressure or temperature of the tires fitted on these wheels, and intended to inform the driver of any abnormal change in the measured parameter.
These monitoring systems are conventionally provided with a sensor mounted on each wheel of the vehicle and comprising a microprocessor and a radio frequency transmitter (or RF transmitter), and a central processing unit for receiving the signals transmitted by the transmitters, comprising a computer incorporating a radio frequency receiver (or RF receiver) connected to an antenna.
One of the problems that has to be resolved with such monitoring systems lies in the obligation to have to associate, with each signal received by the receiver of the central processing unit, an indication concerning the location of the sensor and therefore of the wheel originating this signal, this obligation lasting throughout the life of the vehicle, that is, having to be respected even after wheel changes or, more simply, after the positions of these wheels have been reversed.
At the present time, a first locating method consists in using three low-frequency antennas, each positioned in the vicinity of one of the wheels of the vehicle, and in performing a locating procedure consisting in successively exciting each of these three antennas by transmitting a low-frequency magnetic field.
According to this procedure, the sensor mounted on the wheel located in the vicinity of the excited antenna orders, in response to and for the central processing unit, the transmission of a low-frequency signal comprising an identification code of said sensor, in such a way that the successive excitation of the three antennas leads to the locating of the three sensors mounted on the wheels next to these antennas, and, by deduction, to the locating of the fourth sensor.
The main advantage of such a method lies in the fact that the locating procedure is very fast and results in virtually instantaneous locating after the vehicle has been started up.
However, this solution requires the vehicle to be fitted with three antennas with all the attendant constraints: connecting cables, command amplifiers, etc., such that it proves costly.
This drawback concerning the cost of installation of the means of implementing the locating method can be resolved when the vehicle is fitted with a hands-free access device intended to allow access to said vehicle and the latter to be started up.
In practice, the solution then consists, as in particular described in patent application WO 02/051654, in using the transmitting antennas of this hands-free access device mounted on the vehicle to implement the wheel locating procedure.
As described in the abovementioned patent application, implementing this solution involves, for example, ordering the transmission by the transmitting antennas of an uncoded signal when said antennas are used to locate the wheels, and in ordering the transmission of a coded signal when using these antennas for their original vehicle access control purpose.
Such a theoretically very seductive solution does, however, prove very difficult to implement in practice. In practice, the antennas of the hands-free access devices are not positioned ideally to allow for the locating of the wheels of a vehicle.
Because of this, and firstly, the selective nature of this locating procedure can be obtained only through very accurate settings of the antenna transmitting power, and normally requires the sensitivity of the receivers mounted on the wheels to be increased, consequently sensitizing these sensors to external disturbances.
In practice, these extreme setting conditions often lead to problems of noise immunity and sensitivity tolerance of the sensors, resulting in a very relative reliability of the locating method.